1. Field of the Invention
This specification relates to superparamagnetic nanoparticles, and a method for producing the same. Particularly, the superparamagnetic nanoparticles have osmotic-drawing characteristics, have high dispersibility in water and is recyclable. These nanoparticles can be applied as solutes for water treatment system or for desalinating seawater.
2. Background of the Invention
In order to prepare for the lack or exhaustion of water resources, researches into desalinating seawater or treating wastewater through an osmosis membrane filtration process have currently been conducted. Such water treatment employs a reverse osmosis membrane filtration or a forward osmosis membrane filtration. The reverse osmosis membrane filtration process employs a pressurizing type, which causes high energy consumption. So, in recent time, the forward osmosis membrane filtration process is preferred.
However, the forward osmosis membrane filtration has also primary problems to be solved. One of the problems is found in a method for recovering a solute from an osmotic draw solution after execution of the forward osmosis membrane filtration. Studies of using ammonium carbonate as the solute of the draw solution are actively in progress. This material has an advantage of being recyclable, after used, by being decomposed into gas and then collected. However, this method also has drawbacks of causing considerable energy consumption and generating ammonia gas which is harmful to environments and toxic.
To overcome such drawbacks, a study, in which superparamagnetic nanoparticles dispersible in water are used as solutes to draw a forward osmosis, and a magnetic field is used to collect the superparamagnetic nanoparticles from a draw solution for recycling, has been first reported by Ming M. L. et. al in University for Singapore (Ing. Eng. Chem. Res., 2010, 49, 5869-5876). In regard of this document, a mixture of Fe(acac)3 precursor with 2-pyrrolidine, triethylene glycol, or triethylene glycol/polyacrylic acid was refluxed at high temperature over 245° C. so as to produce iron oxide superparamagnetic nanoparticles which are dispersible in water. Here, the superparamagnetic iron oxide nanoparticles, which were produced from the mixture of Fe(acac)3 precursor and triethylene glycol/polyacrylic acid and had surfaces coordinated with polyacrylic acid, showed the most excellent dispersibility and osmosis. This exhibited 7.5 LMH (L·m−2·hr−1) permeation flux in a primary desalination experiment using salt water. However, several limitations to the recyclability were observed in this document as well. That is, aggregation of the superparamagnetic nanoparticles was caused to thereby increase the size of the nanoparticles recovered under the magnetic field from 21 nm prior to recovery to 50.8 nm after recovery, and accordingly the permeation flux was decreased down to 2 LMH in a secondary desalination experiment. Also, the synthesis of Ming' study is performed using high-priced Fe(acac)3 precursor at the high temperature of 245° C., so it is not economical.
The osmosis increases in proportion to osmolality of a solute which is dissolved or dispersed in water. Hence, materials, which are dissolved or dispersed in water to provide more solutes and easier to be recovered and recycled, exhibit advantages in economical and eco-friendly aspects. Nanoparticles have many number of organic molecules bonded on surfaces thereof, so water-dispersibility of the nanoparticles and osmolality are in a directly proportional relationship up to a critical concentration. The water-dispersibility of the superparamagnetic nanoparticles is determined by a hydrodynamic size in water, and it is preferable for the nanoparticles to have a size smaller than 20 nm and exhibit a monodispersed distribution.
In general, when the size of a magnetic nanocrystal exceeds 20 nm, the nanocrystal has ferromagnetism or ferrimagnetisms, which makes it impossible to control the aggregation of the magnetic nanoparticles. Iron oxide nanoparticles synthesized by the traditional coprecipitation method may have a size less than 20 nm, but be aggregated due to high surface energy with less surface charge. Thus, it has been known that the hydrodynamic size distribution of the magnetic nanoparticles is wide away.
In the meantime, hydrophobic superparamagnetic nanoparticles synthesized in nonpolar organic solution, as recently reported, exhibit a monodispersed distribution in size and are well dispersed in a nonpolar organic solvent, but not dispersed in water. In order for the hydrophobic superparamagnetic nanoparticles to have the water-dispersibility, they must go through a surface modification process, but the surface modification is complicated and non-economical. Also, even after the surface modification, it is difficult to have hydrodynamic monodispersibility. In addition, economic performance and practicability should have priorities in processes, such as desalination of seawater, requiring for a large quantity of materials made with low costs.